EFFECTS OF PARASITIZATION BY COTESIA-CONGREGATA ON THE BRAIN-PROTHORACIC GLAND AXIS OF ITS HOST, MANDUCA-SEXTA

The ability of prothoracic glands (PTGs) from parasitized and unparasi tized Manduca sexta 5th-instars to respond to ecdysiotropic extracts p repared fi om day-5 5th instar brains was compared. An in vitro bioass ay revealed that PTGs from parasitized animals were much less responsi ve to brain PTTH than glands from unparasitized larvae. However, when incubated in Grace's medium in the absence of brain extract, glands fr om day-3 and -4 hosts remained active for a much longer period of time than did those dissected from their unparasitized counterparts. Rathe r than exhibiting reduced (basal) levels of synthesis after the 3rd ho ur of incubation, glands from these parasitized larvae continued to sy nthesize/release ecdysteroid into the medium at relatively high rates. The timing of this enhanced secretory activity is coincident with the ecdysteroid peak that occurs just prior to and during wasp emergence. Following parasite emergence, gland activity decreased, and by the th ird day after emergence; was reduced to low levels. Results suggest th at the requirement for PTTH to stimulate ecdysteroid production has be en bypassed, i.e. that the parasite has uncoupled the normal mechanism s that permit brain regulation of PTG activity. The ability of brains from parasitized M. sexta to stimulate PTGs from unparasitized day-2 5 th instars was also examined. Dose-response analyses performed for the first 7 days of the 5th instar showed that on a per brain basis ecdys iotropic activity in brains from parasitized and unparasitized animals was similar. However, when differences in brain size were considered, ecdysiotropic activity appeared to be more concentrated in brains fro m day-7 parasitized larvae than in brains from similarly aged unparasi tized larvae. Analysis of the size distribution of the ecdysiotropic a ctivity in brains from parasitized larvae revealed a unique form that was larger than the 29 kDa standard. This suggests that parasitization may inhibit neuropeptide processing, particularly during the final st ages preceding emergence of the wasps from the host. Thus, both an inh ibition of prothpracicotropic hormone processing and the inability to respond to this neurohormone may contribute to the developmental arres t characteristic of parasitized 5th instars. Published by Elsevier Sci ence Ltd.